Rubber compositions usable in tire treads and having improved processability have been described in U.S. Pat. No. 3,795,651. The composition comprises at least 30 percent by weight of styrene-butadiene random copolymer which has a Mooney viscosity of from 40 to 150 and a relaxation time of from 20 to 200 seconds at 100.degree. C. at a viscometer rotor rate of 2 rpm. The relaxation time is defined as the time following normal measurement of the Mooney viscosity for the viscometer reading to reach a value of 20 percent of the Mooney viscosity value immediately prior to stopping the viscometer rotor.
An article appearing in Plastics and Rubber: Processing (September, 1977) entitlted "Assessement of rubber processability" noted that evidence exists to establish that processing problems in rubber products arise directly from variability in raw polymer characteristics, particularly molecular weight and the distribution thereof. Upon testing raw natural rubber, it was reported that Mooney viscosity was related to the weight average molecular weight while relaxation time was related to the molecular weight distribution.
Notwithstanding the foregoing art, until the present invention it has not been known to utilize stress relaxation time of a raw polymer to determine and control the Mooney viscosity of the rubber compound which, in turn, reduces variation in the processability of the compound.